feat(api): add tuning params (temperature, top_p, penalties, stop) to MessageRequest

MessageRequest was missing standard OpenAI-compatible generation tuning
parameters. Callers had no way to control temperature, top_p,
frequency_penalty, presence_penalty, or stop sequences.

Changes:
- Added 5 optional fields to MessageRequest (all Option, None by default)
- Wired into build_chat_completion_request: only included in payload when set
- All existing construction sites updated with ..Default::default()
- MessageRequest now derives Default for ergonomic partial construction

Tests added:
- tuning_params_included_in_payload_when_set: all 5 params flow into JSON
- tuning_params_omitted_from_payload_when_none: absent params stay absent

83 api lib tests passing, 0 failing.
cargo check --workspace: 0 warnings.

rust/crates/api/src/prompt_cache.rs

@@ -704,6 +704,7 @@ mod tests {
             tools: None,
             tool_choice: None,
             stream: false,
+            ..Default::default()
         }
     }